In general, hot materials 110 such as slabs and billets are heated up to a predetermined temperature while being carried in a reheating furnace 100 before they are hot rolled. As shown in FIG. 1, the reheating furnace 100 contains a skid apparatus 120 for supporting and carrying the hot materials 110 within the reheating furnace 100, a plurality of burners 122 functioning as heat sources and an exhaust duct unit 130 for exhausting atmospheric gas out of the reheating furnace 100.
The skid apparatus 120 includes stationary beam skids 124 and movable beam skids 126 for moving the hot materials 110, in which the movable beam skids 126 carry out a transport cycle including elevation, advancement, descent and retreat to carry the hot materials 110 within the reheating furnace 100 toward an exit while the stationary beam skids 124 support the hot materials 110.
The skid apparatus 120 has coolant pipes 140 which are placed on top portions of the skid beams, as shown in FIGS. 2 and 3, each surrounded by a heat insulation layer 142 for allowing passage of coolant through the same. On the coolant pipes 140, there are mounted a plurality of skid members 150 made of ceramic composite or special refractory steel for supporting the hot materials 110.
Each of the skid members 150 may be in the form of a column having a polygonal cross-sectional configuration, such as a hexagonal section as shown in FIG. 4a, a circular cross-sectional configuration as shown in FIG. 4c or a quadrangular cross-sectional configuration as shown in FIG. 4d. Also, as shown in FIG. 4b, the skid member 150 may be shaped as a circular column having heat-absorbing fins 150a mounted on the top thereof.
As shown in FIG. 5a, the skid member 150 may have a stationary rail structure extended longitudinally along the coolant pipe 140, which is suitable for pushing the hot materials into the furnace through an entrance so that the hot materials are slid on the stationary rail.
Also as shown in FIGS. 5b and 5c, the skid member 150 comprises assembling structures 143 projected from the coolant pipe 140 and a rider 144 coupled with the assembling structures 143 along the length of the coolant pipe 140.
In the skid apparatus 120 of the conventional reheating furnace 100, the skid members 150 are cooled down with coolants such as cold water or steam, etc., while they support the hot materials 110. As a result, when a top portion 161 (FIG. 6B) of each skid member 150 supports an underside region of each hot material 110, the underside region is cooled down with coolant to form a skid mark 160 having a temperature lower than other regions of the hot material 110. That is, the skid mark 160 is formed in the underside region of the hot material 110 contacted with the top portion 161 of the skid member 150, in which the contacted underside region of the hot material 110 has a temperature lower than other regions of the hot material, thereby creating temperature difference to the hot material.
Therefore, the prior art maintains the temperature difference of about at least 20 to 30° C. between the skid mark 160 and other regions of the hot material. Since the skid mark 160 has such a large value of temperature difference, this causes difference of elongation to the hot material 110 in a subsequent hot rolling process thereby degrading precision of rolling thickness and width.
Such degraded precision of rolling thickness creates a localized thick portion to the hot material in a finishing mill process which is requiring a precise rolling control of thickness, this is caused by the increase of deformation resistance at the skid mark of low temperature while the hot material is rolled under tension between hot rolling stands.
On the contrary, when the hot finishing mill process is performed to a steel strip or plate of ultra low carbon content in a temperature range (860 to 890° C.) below the transformation point (Ar3: about 910° C.) where a skid mark region undergoes phase-transformation (e.g., from austenite to ferrite), deformation resistance decreases rapidly in the longitudinal direction of rolled steel to worsen rolling threading ability or to reduce strip or plate thickness radically thereby tearing them off.
If the temperature of the hot rolled strip or plate is raised to avoid the foregoing problems, energy is consumed excessively. Also, the surface of the hot rolled strip or plate may be scaled and rolls suffer from heavy thermal fatigue.
Therefore, in the prior art, heating time is prolonged or the temperature of the reheating furnace 100 is raised excessively in order to decrease the temperature difference associated with the skid mark 160. However, temperature rise in the reheating furnace 100 consumes fuel by a large quantity thereby raising the production cost of steel products with the reheating furnace 100. Also, the hot material is overheated thereby increasing scales as well as lowering the yield of steel products. Further, even though the scales formed on the surface of the hot material are removed via impact of high-pressure water, some of scales remain on the surface causing surface defects to the rolled hot material.
In order to prevent problems related with hot rolling and subsequent processes, it is required to maintain the temperature difference associated with the skid mark 160 within about 20° C., preferably, about 18° C.
Several improvements have been proposed in the prior art, to solve the problems related with subsequent processes caused by the skid mark.
Japanese Laid-Open Patent Publication Serial No. H2-85322 discloses a laser apparatus capable of detecting the temperature of a skid mark in a rolled slab and emitting a laser beam to the skid mark from the exit side of a reheating furnace to further heat the skid mark so that the temperature of the skid mark rises equal to that of the slab. Since the laser apparatus is provided in addition to the reheating furnace, this technique requires additional cost for the laser apparatus.
Japanese Laid-Open Patent Publication Ser. Nos. H3-207808 and H5-179339 propose techniques for mounting a skid mark burner in the exit side of a reheating furnace to heat a corresponding region of a slab to remove any skid mark from the slab and to provide the skid mark burner with excellent endurance. According to these techniques, the burner is installed within the reheating furnace to be used exclusively for the skid mark, and the burner also increases installation cost.
Japanese Laid-Open Patent Publication Serial Nos. H3-47913 and H4-131318 disclose a skid button. This skid button has an internal space and is partitioned into two or three vertical sections, in which an upper section is made of a material excellent in heat conductivity and a lower section is made of material excellent in endurance and structural strength. However, such partitioned skid button is structurally unstable, and causes high fabrication price thereby raising cost.
Also, Japanese Patent Publication Serial No. H4-57727 discloses a cylindrical skid member within a skid member holder on a skid coolant pipe, in which the skid member is made of heat insulation material such as non-oxide ceramic and has a hollow space or an upward opening. However, this skid member also has a partitioned structure and thus disadvantageously increases installation cost. Further, scales are deposited in the opening to fill the same, resultantly giving an effect of filling the opening with insulation material.
Japanese Laid-Open Patent Publication Serial No. H6-306453 discloses an apparatus comprising a burner installed in a lower portion of the exit side of a reheating furnace, a local heating controller for controlling the burner and a time predicting controller in order to minimize the temperature difference between a skid mark and other regions of a slab based upon the temperature of the skid mark. This apparatus also requires extension of equipment installation.
Another Japanese Laid-Open Patent Publication Serial No. H9-268314 discloses skid button having a cylindrical short pipe installed on a skid member holder extended from a skid pipe, in which refractory castable is filled into the pipe remaining a gap in an upper portion thereof. However, since the short pipe reduced in sectional area only supports the contact region of a slab, a large quantity of surface pressure is applied to the contact region of the slab potentially leaving a mark on the contact region of the slab. In this skid button, while the gap and the refractory castable block heat generated from the slab to prevent creation of a skid mark in an initial stage, as the time goes by, scales are deposited in the gap filling the same thereby disabling the effect of the gap to a certain degree.
Japanese Laid-Open Patent Publication Serial No. H10-140246 proposes an apparatus which comprises a water cooling pipe arranged inside a skid beam under a tempering zone of a reheating furnace and an auxiliary heating gas line incorporating a gas supplying pipe extended upwardly through a refractory layer and having a nozzle placed right below a slab for heating a skid mark of the slab. This apparatus can remove the skid mark through localized heating to the slab, but needs extension of installations thereby causing rise in equipment price and running cost.
Another Japanese Laid-Open Patent Publication Serial No. H10-140247 installs a plurality of regenerative burners above a tempering zone in a reheating furnace to further heat a skid mark on a slab thereby reducing the temperature difference between the skid mark and other regions of the slab. However, this technique requires additional regenerative burners thereby raising installation cost by a large quantity as well as manufacturing cost through additional heating.
Japanese Laid-Open Patent Publication Serial No. H10-306313 discloses a technique for heating skid beams with fuel supplying pipes installed in one of the skid beams thereby to prevent a skid mark in an underside region of a slab supported by the skid beams. Since this technique of the prior art also requires the fuel supplying pipes on the skid bear, there are problems that extension of installations consumes a large amount of cost as well as complicates a system design.
Furthermore, Japanese Laid-Open Patent Publication Serial No. 2000-61503 provides a solenoid-induced heating apparatus between a prime rolling mill and a finishing mill to heat a low temperature region of a slab over other regions thereof. This conventional technique also needs additional heating units.
The present invention has been made to solve the foregoing problems of the prior art and it is therefore an object of the invention to provide an improved method and a skid member for reducing temperature difference in a hot material to be heated and a skid apparatus using the same, more particularly, which reduces the heat transfer area from the hot material toward a lower portion of the skid member and forms a ventilation channel enlarging an area in contact with hot gas to increase the quantity of heat introduced into the skid member, thereby reducing heat loss from an upper portion of the skid member to the lower portion thereof with the ventilation channel and thus imparting compensatory heating to the upper portion of the skid member.
It is another object of the invention to provide an improved method and a skid member for reducing temperature difference in a hot material to be heated and a skid apparatus using the same, more particularly, which reduces the temperature difference between a skid mark and other regions of a hot material to be heated through simple structural improvement so that the hot material can be heated at a uniform temperature to improve the hot rolling threading ability and quality of the hot material in subsequent processes.
It is yet another object of the invention to provide an improved method and a skid member for reducing temperature difference in a hot material to be heated and a skid apparatus using the same, more particularly, which introduces hot gas within a reheating furnace into the skid member while maintaining the contour of the skid member to reduce temperature difference associated with a skid mark, thereby improving percentile thickness and width within tolerances and restraining creation of scales to minimize descaling operation, improve rolling yield and save manufacturing cost.